Supported bimetallic catalysts derived from molecular metal clusters

The use of structurally well-defined organometallic clusters as catalyst precursors provides the means to obtain fundamental understanding of supported bimetallic catalysts. This investigation is focused on the role of a Group VII metal (Re) in modifying and stabilizing Group VIII metals (Os or Pt) in hydrocarbon conversion catalysts; the catalyst precursors were (H{dollar}sb3{dollar}ReOs{dollar}sb3{dollar}(CO){dollar}sb{lcub}13{rcub}{dollar}) and (Re{dollar}sb2{dollar}Pt(CO){dollar}sb{lcub}12{rcub}{dollar}). In-situ infrared, X-ray photoelectron, temperature-programmed reduction, and extended X-ray absorption fine structure spectroscopies were employed to follow the structural evolution of the surface species made from these bimetallic precursors as well as their monometallic counterparts. Hydrocarbon conversion reactions were used to assess the catalytic properties of the samples.; Catalysts formed from (H{dollar}sb3{dollar}ReOs{dollar}sb3{dollar}(CO){dollar}sb{lcub}13{rcub}{dollar}) on {dollar}gamma{dollar}-alumina were characterized by IR and XP spectroscopies and a catalytic test reaction ({dollar}n{dollar}-butane hydrogenolysis). The results indicate that the catalyst consists of Os crystallites promoted by low-valent Re ions; the Re ions lead to increased catalytic activity and stability, although the supported rhenium alone lacks catalytic activity. The known surface chemistry of Os on basic magnesia was used to obtain a better understanding of the Re-Os bimetallic interaction. Addition of Re suppressed the formation of the catalytically inactive and highly stable (Os{dollar}sb{lcub}10{rcub}{dollar}C(CO){dollar}sb{lcub}24{rcub}{dollar}) {dollar}sp{lcub}2-{rcub}{dollar} on the MgO surface and resulted in a catalyst having greater stability for CO hydrogenation than the catalyst incorporating Os alone.; Another goal of the work was to investigate the structure-catalytic activity relationships for alumina-supported Pt-Re catalysts used for naphtha reforming. There are some unique advantages of cluster-derived catalysts, including the high metal dispersion and exact stoichiometric composition, giving relatively simple metal structures. EXAFS spectra of a H{dollar}sb2{dollar}-treated (Re{dollar}sb2{dollar}Pt(CO){dollar}sb{lcub}12{rcub}{dollar}) -derived catalyst were measured to characterize the metal-support and metal-metal interfaces. A detailed analysis of both the Re and the Pt L{dollar}sb{lcub}III{rcub}{dollar} edges of the X-ray absorption spectra forms the basis for a novel "bilayer" structural model. The coordination parameters suggest that Pt is present on top of a rhenium layer on the alumina forming, on average 8 A ensembles, in agreement with the results of IR, XPS, and TPR. A simple deactivation kinetics model was developed to describe the improved activity maintenance of the (Re{dollar}sb2{dollar}Pt(CO){dollar}sb{lcub}12{rcub}{dollar}) -derived catalyst over that of salt-derived Pt and Pt-Re catalysts for a model reforming reaction (methylcyclohexane dehydrogenation). High Pt dispersion and a small catalyst decay rate constant are identified as important for the catalyst stability, and the literature evidence of bilayers in alumina-supported Pt-Ir and Pt-Sn catalysts points to a more general structural role of the non-platinum metal in the activity maintenance by Pt-containing bimetallic reforming catalysts.